Most cats will be infected with an internal parasite at some point in their lives, most likely as kittens. The most common parasites that a cat faces are roundworms, hookworms and tapeworms. These can be life-threatening and must be treated quickly. A veterinarian can prescribe common medication that will rid your cat of these parasites. If you prefer to try a home remedy, there are a number to choose from--but always consult with your veterinarian first.
Add food-grade diatomaceous earth to all of your cat's meals. Diatomaceous earth is a sedimentary rock that can be ground to a powder and sprinkled on food. It is often used as an insecticide because it absorbs lipids from insects and kills them through dehydration. Many farmers use it to deworm livestock. Sprinkle about an ounce on each meal.
Mix fresh, minced garlic into your cat's food once a day. Though it may act more as a preventive, garlic might help kill present parasites as well. The only trouble might be getting your cat to eat its food with the slight change in flavour. Start with very small amounts of garlic--just a pinch--and increase it as your cat grows more used to it.
Put your cat on a one-day fast. If your cat is otherwise healthy, a 24-hour fast can help clear out parasites, which cannot last as long without food. This can be done once a week without threat to your kitty's health.
Grind up raw pumpkin seeds and mix them in with one meal per day. This is especially effective against tapeworms. Grind the seeds fresh every day, rather than grinding a large amount and saving it for use throughout the week. About 1/4 tsp is all you need for each meal.
Keep your cats inside to prevent them from being infected. Clean litter boxes every day, and empty and wash them once a week.
Never use any treatment before talking with a veterinarian. Although all of the above treatments are harmless, they may not be enough on their own to get rid of a cat's parasites.